Separation/concentration of liquid mixtures by a membrane separation method is an energy-saving method as compared with a separation technique such as distillation because it is not accompanied with phase change. Also, it does not cause the phase changes in the state of substances. Therefore, it has been widely utilized in many fields including a food field such as concentration of fruit juice and separation of beer enzyme; and recovery of organic substances from industrial wastewater. Treatment of water by a semipermeable membrane has become firmly established as an indispensable process supporting the most leading-edge technology.
For example, generation of energy by means of pressure and flow rate achieved by utilizing the difference in concentrations between seawater and freshwater with a semipermeable membrane is a clean process and has been expected as a recyclable energy. Particularly due to the fact that a hollow fiber type semipermeable membrane can make the membrane area per membrane module volume large in spite of its small permeation flow rate per unit membrane area as compared with a spiral wound type semipermeable membrane, it has advantages that the permeation flow rate can be made large as a whole and that volume efficiency is very high whereby it has excellent compactness. Further, when both aqueous solution of high concentration and freshwater are fed into a module and made to contact with each other via a hollow fiber type semipermeable membrane, concentration polarization on the membrane surface can be made small.
Generally, such hollow fiber type semipermeable membrane is manufactured in such a method that a dope containing cellulose acetate is prepared as a polymer material, extruded from a nozzle into air, coagulated in a coagulating solution, washed and shrunk by annealing by hot water. As to a semipermeable membrane for keeping both water permeability and separating property in high level, those mentioned in Patent Documents 1 and 2 are exemplified. Patent Document 1 discloses an art concerning a hollow fiber type semipermeable membrane module utilized for separation of solid or separation of solute from a liquid mixture. However, according to the properties of the hollow fiber membrane using cellulose triacetate shown in Table 1 of Patent Document 1, the permeation flow rate (FR1) measured under operating pressure of 55 kg/cm2 is 22.6 to 91.5 l/(m2·day) whereby no high water permeability can be achieved. In addition, water permeability utilizing the concentration difference as a driving force is not disclosed. Moreover, the membrane mentioned in said document has been treated at a high annealing temperature whereby the membrane structure thereof is densified and it is not suitable for membrane separation utilizing the concentration difference as a driving force. Particularly when it is used for generation of energy under such a condition that the high concentration side is in a pressurized state, the effective pressure difference [(osmotic pressure difference)−(pressure at high concentration side)] wherein the concentration difference is a driving force becomes the result of subtraction of the pressure at high concentration side in the pressurized state from the osmotic pressure difference whereby the permeation flow rate becomes low. Further, since the membrane is used in the pressurized state, the pressure resistance corresponding to said pressurized condition is necessary.
Patent Document 2 discloses an art concerning a flat sheet type composite semipermeable membrane which is equipped, on a microporous support, with an active layer (thin membrane, skin layer) containing interfacial polymerized polyamide as a main component and which has both high salt rejection rate and high water permeability when pressure is utilized as a driving force. According to the description of Example 1, the semipermeable membrane disclosed in the Patent Document 2 has permeation flow rate of 1.0 m3/m2·day (1000 L/m2·day) when measured under an operation pressure of 7.5 kg/cm2. However, this semipermeable membrane is in a form of flat membrane whereby, when it is used for the actual water treatment wherein the concentration difference via semipermeable membrane is utilized as a driving force, it is difficult that aqueous solution of high concentration and aqueous solution of low concentration (freshwater) to be fed are effectively and uniformly partitioned to the surface of the membrane. In addition, in the area to which small flow rate of the aqueous solution is fed, concentration polarization on the membrane surface becomes particularly high. Accordingly, it is difficult to ensure the effective concentration difference whereby there is a disadvantage that efficiency of the water treatment cannot be made high. Moreover, in the membrane comprising such a polyamide material, there are disadvantages that it is inferior in its resistance to chlorine and that usable bactericidal chemicals are limited.
On the other hand, there has been a brisk demand from the consumers paying their importance to economy and compactness of a water treatment membrane plant for improving the treating ability per membrane area in a hollow fiber type semipermeable membrane. In the case of water treatment wherein the concentration difference is utilized as a driving force without application of high pressure, no high water permeability is achieved even if the conventional semipermeable membrane for lower pressure is used. As a result, it is the present situation that water production cost and installation space cannot be suppressed.
As mentioned hereinabove, it is the present situation that there is no hollow fiber type semipermeable membrane which can achieve both water permeability and selectivity in high level and which can perform efficient water treatment utilizing the concentration difference as a driving force, in a small installation space.